Is this correct? I often see "how many pairs of underwear", but this doesn't make sense. How is underwear a pair? There is only 1. I've googled and I've seen both "how much underwear" and "how many pairs of underwear". Which one is correct?
Underwear, like trousers or jeans, are referred to as a pair, because it's a throwback to when pants (pantaloons) originally came in two pieces - a matching pair. A person would put on one leg, tie it around their waist, then put on the other leg and do the same.
Before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg. The pieces were put on each leg separately and then wrapped and tied or belted at the waist (just like cowboys’ chaps). The plural usage persisted out of habit even after the garments had become physically one piece. However, a shirt was a single piece of cloth, so it was always singular.
Pants have obviously evolved, but the terminology still remains.
Therefore, "how many pairs of underwear" is correct usage. For example:
How many pairs of underwear are you packing for the trip?
I'm bringing five pairs of underwear with me.
That being said however, in question form, we can use much, e.g.
How much underwear should I bring?
you should bring five pairs of underwear.
It's perhaps worth adding that there is a discrepancy between American English and British English here. While in the former "underwear" can be used to mean a specific garment, so "pairs of underwear" makes sense for the reasons Mike gives, in British English "underwear" only has the more general meaning, and so a BrE speaker might say either "how much underwear" or "how many pairs of underpants" (or just "pairs of pants", which in BrE only refers to underwear).
The word “underwear” is a mass noun that takes singular agreement (“your underwear is showing”) but there are a cluster of pluralia tantum underwear words. 11 Nouns That Only Have a Plural Form
Anyway, both are commonly used:
Then how much underwear and how many pairs of socks shall I put in? Last Watch
What type of place would tell you how many pairs of underwear to bring? The Boy with the Lampshade on His Head
Many and much are used according to what sort of a noun they are referring to. There are two types, and they are called mass nouns and count nouns.
These can be counted. Birds, apples, cell phones, etc.
These nouns are accompanied with "many".
- I see many birds on the horizon.
- The grocer sold many apples yesterday.
- My science professor has too many cell phones.
These occur in quantities that are measured rather than counted. Water, sugar, abstract ideas, etc.
These nouns are accompanied with "much".
- There is much water in the ocean.
- How much sugar would you like?
- You argue too much!
Which category do you feel "underwear" belong to?
I would say, "How many pairs of underwear should I bring", or simply "How many underwear should I bring?"